1. Field of the Invention
This invention relates to a method and an apparatus for reproducing a signal of a run-length-limited code from a recording medium such as an optical disc. In addition, this invention relates to an apparatus for recording and reproducing a signal of a run-length-limited code on and from a recording medium such as an optical disc.
2. Description of the Related Art
Japanese patent application publication number 10-106161/1998 discloses an optical information reproducing apparatus based on a PRML (partial response maximum likelihood) system. In the apparatus of Japanese application 10-106161, information of a run-length-limited code is reproduced from an optical disc through a reproducing section, and a transversal filter subjects the reproduced waveform to partial-response equalization depending on tap coefficients. An example of the partial-response equalization is PR(1, X, X, 1) equalization. The output signal of the transversal filter is decoded into binary data by a maximum-likelihood decoder. A parameter setting device selects intersymbol-interference imparting values in the partial-response equalization in accordance with the characteristics of the reproduced waveform. Furthermore, the parameter setting device sets the tap coefficients of the transversal filter and a decision point signal level for the maximum likelihood decoder as parameters in response to the selected intersymbol-interference imparting values.
In the apparatus of Japanese application 10-106161, the parameter setting device includes a memory loaded with parameter-setting reference data. A target after-equalization waveform is determined on the basis of the parameter-setting reference data and the selected intersymbol-interference imparting values. The apparatus of Japanese application 10-106161 premises that the optical disc has predetermined pits (reference pits) representative of the parameter-setting reference data. A reference-data signal is reproduced from the predetermined pits in the optical disc. The tap coefficients of the transversal filter are set to equalize the waveform of the reproduced reference-data signal to the target after-equalization waveform.
In the apparatus of Japanese application 10-106161, the parameter setting device also includes an error-rate deciding device which compares the binary data outputted from the maximum-likelihood decoder and the parameter-setting reference data fed from the memory to calculate the rate of bit errors therebetween. The error-rate deciding device judges whether or not the calculated bit error rate is within an allowable range. The selection of intersymbol-interference imparting values are responsive to the result of the judgment by the error-rate deciding device. The tap coefficients of the transversal filter and the decision point signal level for the maximum-likelihood decoder which occur when the calculated bit error rate is within the allowable range are actually used in the partial-response equalization and the decoding procedure.
In the case where an optical disc stores a digital signal, a signal read out from the optical disc has an analog waveform representative of the digital signal. Generally, a phase locked loop (PLL) circuit is used to reproduce a bit clock signal from the read-out signal. A typical digital signal recorded on an optical disc has various-run-length segments including short-run-length segments, mid-run-length segments, and long-run-length segments. An example of the short-run-length segments is a 2T segment, where T denotes a bit period. As the density of digital information recorded on an optical disc is higher, there occurs a lower level of a signal read out from the optical disc which corresponds to a short-run-length segment (for example, a 2T segment) of the digital information. A very low signal level makes it difficult for a PLL circuit to accurately reproduce a bit clock signal from the read-out signal. In such a case, the PLL circuit moves out of a state locked up with respect to the read-out signal, and hence a bit slip tends to occur. Due to the bit slip, some bits of the digital information are lost along a time base. For example, bits of the digital information continue to be lost until the PLL circuit returns to the locked-up state.
Viterbi decoding includes MAP (maximum a posteriori probability) decoding or SOVA (soft output viterbi algorithm) decoding. The viterbi decoding has the ability to correct errors in a bit stream reproduced from an optical disc. Specifically, the viterbi decoding finds the likeliest stream of bits from a signal read out from the optical disc. Once bits have been lost due to a bit slip, even the viterbi decoding can not recover the lost bits. Thus, the bit slip increases the error rate.
A usual digital signal recorded on an optical disc has sync signals placed at the heads of signal blocks. In the event that a bit slip occurs during the read-out of the digital signal from the optical disc, each reproduced bit position moves out of correct one. Then, the reproduced bit position can be returned to correct one when a sync signal is detected in the read-out signal. In this case, the whole of a signal block immediately preceding the detected sync signal fails to be recovered. That signal block is recognized as an error.
Generally, a digital signal undergoes block-by-block modulation (encoding) before being recorded on an optical disc. Examples of the block-by-block modulation are D4-6 modulation, 8-16 modulation, and 1-7 pp modulation including a run length limiting process and a DSV (digital sum variation or digital sum value) control process. During the reproduction of the digital signal from the optical disc, the reproduced digital signal undergoes block-by-block demodulation (decoding) inverse with respect to the modulation. In the event that a bit slip occurs, the demodulation can not correctly recover some bits of the digital signal.